This invention relates generally to the field of optical disc technology and more particularly, but not by way of limitation, to the placement and retrieval of data on an optical disc using alternate data modulation schemes to facilitate disc authentication efforts, including copy protection and forensic tracking.
Optical discs have become increasingly popular as efficient and cost-effective storage media for digitally stored data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands (also sometimes referred to as xe2x80x9cmarksxe2x80x9d and xe2x80x9cspacesxe2x80x9d) along a continuously extending spiral track.
The data are recovered from the disc through the use of a light source (such as a laser) which applies light of selected wavelength to the rotating disc, and a transducer which generates a readback signal indicative of the data in relation to the relative differences in reflectivity of the pits and lands. It is common to separate the relative elevations of the pits and lands by a distance equal to a quarter-wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands.
One popular optical disc format is commonly referred to as compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CDs). A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 650 megabytes (MB). Another popular optical disc format is commonly referred to as digital versatile disc, or DVD. A DVD has the same form factor as a CD, but has a data storage capacity of about 4.7 gigabytes (GB) per layer. This increased storage capacity generally accommodates a feature-length video movie (DVD-video), or large amounts of other types of data (DVD-ROM, DVD-RAM, DVD-audio, etc.). Other optical disc configurations with different form factors and storage capacities have also been proposed and commercialized.
As will be recognized, the writing of data to an optical disc includes the encoding of each input group of data bits into a corresponding group of modulated bits. Such modulation is necessary to meet run-length requirements that specify how close and how far apart successive logical ones (pit/land transitions) can occur on the disc. If successive transitions are too close together, the detection circuitry may not be able to distinguish the individual transitions; if successive transitions are too far apart, the detection circuitry may lose frequency lock and thus be unable to recover the data from the disc.
Both CDs and DVDs utilize run-length requirements of at least two zeros and no more than ten zeros between successive ones. This results in pits and lands with symbol lengths that range from 3T to 11T, with T of determined period length.
CDs typically use what is referred to as 8/14 modulation (xe2x80x9cEFMxe2x80x9d) so that each 8 bits of input data are modulated into 14 bits of modulated data. For example, an 8-bit data pattern 00001010 (representing the number 10) can provide a corresponding 14-bit modulation pattern of 10010001000000. Since 8/14 modulation patterns (words) can start and/or end with a logical one, CDs additionally use three merge bits between successive modulation words to ensure the run-length rules are not violated at transitions between words. Hence, CD modulation is also sometimes referred to as 8/17 modulation.
DVDs typically use what is referred to as 8/16 modulation (xe2x80x9cEFM+xe2x80x9d) so that each 8 bits of input data are modulated into a corresponding 16-bit modulation pattern. For example, the 8-bit pattern 00001010 (representing the number 10) can result in a corresponding 16-bit modulation pattern of 0010010001000000. Unlike CDs, no merge bits are required between adjacent DVD modulation words.
Due to the worldwide consumer demand for the types of information available on optical discs (e.g., software, music, video, etc.), combined with the relative ease with which unauthorized copies of optical discs can be generated, suppliers of optical discs have attempted to incorporate schemes to authenticate the discs; that is, to determine whether a particular disc is an authorized copy. Such disc authentication can occur in the form of copy protection or forensic tracking efforts.
Generally, copy protection involves configuring the optical disc in such a manner that an authorized disc functions properly in a readback system, but an unauthorized copy does not. One common copy protection scheme used in CD-ROM discs is to write certain copy protection data at certain locations on the disc. When the CD-ROM is subsequently loaded into a host computer, a system processor polls these locations and verifies the contents before granting access to remaining portions of the disc.
Forensic tracking efforts generally entail storing certain xe2x80x9cmarkingxe2x80x9d or xe2x80x9ctrackingxe2x80x9d information on the disc. The forensic information usually does not prevent an unauthorized copy from functioning in the readback system, but based on the presence or absence of the information, a determination can be made whether a particular optical disc is in fact an authorized copy. Forensic information may include, for example, information relating to the ownership of the contents of the disc, when and where the mastering process occurred, etc.
Various ways to embed data in an optical disc for use in disc authentication efforts have been proposed in the prior art. A particular approach involves the creation of certain disruptions in the optical disc by violating the run length requirements of the modulation scheme used to record data to the optical disc. For example, in the case of the 8/14 CD and 8/16 DVD modulation schemes which only allow lengths of 3T-11T, embedded data can be incorporated having symbols that are less than 3T or greater than 11T in length.
There are several problems with this approach, however. The run-length requirements of a given modulation scheme are there to ensure the readback system can properly detect and decode the data. Hence, the optical tracking system may have trouble detecting symbol lengths that are too short and the recovery tracking oscillators may lose frequency lock if the symbols are too long. If the run-length violations are sufficiently severe, the violations can induce uncorrectable errors in the application data, compromising the integrity of the optical disc itself.
More importantly, such run-length violations are easily detectable using standard time interval analysis (TIA) test equipment, allowing unscrupulous parties to readily duplicate and defeat the copy protection or forensic information scheme altogether.
Because of these and other considerations, there remains a continued need in the art for improved ways to embed data on an optical disc for disc authentication purposes without interfering with the operability of the disc. It is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for writing data to an optical disc, and for subsequently reading the data therefrom.
In accordance with preferred embodiments, a first modulation encoding scheme (MES1) is defined for the encoding of input data as data symbols having nominal symbol lengths over a first range of symbol lengths. The MES1 is generally related to the particular type of disc; for example, a CD will typically use 8/14 and a DVD will use 8/16.
A second modulation encoding scheme (MES2) different from the first modulation encoding scheme is also defined for the encoding of input data as data symbols having nominal symbol lengths over a second range of symbol lengths. The MES2 is selected to have the same (or to have more stringent) run-length rules than the MES1, so that each symbol length of the second range of symbol lengths from the MES2 appears in the first range of symbol lengths MES1. Moreover, the MES2 is configured to output different symbols for a given input word than the MES1.
The MES1 is used to encode and write primary data to the optical disc, with the primary data having an informational content used by a user of the optical disc. At selected times during the writing of the primary data, the MES2 is used to encode and write secondary data to the optical disc at a location adjacent a portion of the primary data. The secondary data have an informational content used to authenticate the optical disc as an authorized copy and include a leading synchronization pattern which, when detected, allows remaining portions of the secondary data to be decoded using the MES2.
Preferably, the data are subsequently read from the optical disc using a specially configured readback system having a primary decoder configured in accordance with the MES1 to decode the primary data, and a secondary decoder connected in parallel with the primary decoder configured in accordance with the MES2 to decode the secondary data. A readback modulation signal is transduced from the optical disc and concurrently transmitted to both the primary and secondary decoders. The primary decoder operates upon the readback modulation signal to output the primary data, while the secondary decoder searches the readback modulation signal for the leading synchronization pattern and, upon detection, outputs the secondary data. Preferably, the secondary decoder is further configured to detect and correct errors in the secondary data.
In this way, disc authentication data can be readily placed on the optical disc without interfering with the readback capabilities of the disc. Because the encoding of the secondary data does not violate the standard run-length rules for the disc (i.e., the MES1), no run-length violations will be detected.
These and various other features and advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.